Arthroplasty procedures, such as a total hip replacement, can require the removal of the femoral head and neck, followed by implantation of an artificial hip prosthesis into a reamed portion of the femoral medullary canal.
In order to ensure that they are strong and durable, hip prostheses are constructed of metal or alloys thereof. As a result of their metal construction, hip prostheses have a significantly greater stiffness than both the medullary bone canal into which they are implanted and the bone which supports them. Because of this stiffness difference, implanted hip prostheses tend to cause such undesirable effects as cortical bridging or thickening, myositis ossificans and stress shielding.
Stress shielding is an especially undesirable effect of metal hip prosthesis implantation for two reasons. First, stress shielding usually causes associated thigh pain in the implantee. Second, stress shielding can lead, over time, to resorption of the femoral bone which, in turn, can prematurely necessitate the total replacement of a hip prosthesis via a revision process.
Those in the art, in an effort to alleviate the problem of stress shielding arising from an implanted hip prosthesis, have unsuccessfully attempted to provide a hip prosthesis that has bending and strain characteristics such that the bone which supports the implanted prosthesis is able to grow strong while, at the same time, the prosthesis is capable of providing enough structural strength to bear large femoral loads placed upon it for a number of years.
One recent patent, U.S. Pat. No. 5,725,586, describes a bone prosthesis that has a hollow stem. This bone prosthesis, when implanted, is likely to greatly reduce the occurrence of stress shielding because of the increased bending compliance that is provided by its hollow stem.
However, although it works for this intended purpose, the hip prosthesis of U.S. Pat. No 5,725,586 may not provide, due to its design, an optimal amount of structural strength. One of the highest stress concentration points on an implanted hip prosthesis is at the isthmus region of the of the stem of the prosthesis. A hip prosthesis that is either wholly or partially hollow at or distal to the isthmus region of the stem, as is the hip prosthesis of U.S. Pat. No. 5,725,586, may not be strong enough to prevent fracture of the prosthesis at the isthmus region under very large fatigue loads.
Also, a hip prosthesis, such as the one disclosed in U.S. Pat. No. 5,725,586, that is hollow into the distal region of the prosthesis cannot safely allow for the furcation of all or part of its distal region. The prosthesis, if both hollow and furcated in the distal region of the prosthesis, would be especially weak and highly susceptible to an ingress of fluids.
Therefore, it remains a goal in the art to design and implement a hip prosthesis that provides enough structural strength to bear loads that are produced from the hip joint and centered on the femur, and that also provides enough bending compliance to effectively transfer loads and strain to the bone supporting the prosthesis without producing stress shielding and/or other undesired effects.